Plate for electrostatic electro-photography



March 29, 1966 J. L. STOCKDALE 3,243,293

PLATE FOR ELECTROSTATIC ELECTROPHOTOGRAPHY Filed March 26. 1965 'lllllllllllllllllllllfl INVENTOR.

JERRY L. STOCKDALE BYQL United States Patent 3,243,293 PLATE FOR ELECTROSTATIC ELECTRO- PHOTOGRAPHY Jerry L. Stockdale, Indianapolis, Ind., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Mar. 26, 1965, Ser. No. 443,149 5 Claims. (Cl. 961) This invention relates ot xerography and in particular to an improved xerographic plate; and this application is a continuation-in-part of my applications Serial No. 235,383, filed July 5, 1951 (now abandoned), and Serial No. 118,626, filed June 21, 1961.

In the art of xerography, which was first presented in Carlson US. Patent No. 2,297,691, an electrophotographic reproduction is made by placing an electrostatic charge pattern of electrostatic image on an insulating surface such as a photoconductive insulating surface and thereafter developing the electrostatic image by means of an electrically-attractable material.

This invention is an improvement in the art of xerography as disclosed in the aforesaid Carlson patent of such significant character that it has now become feasible to apply the process to continuous-tone electrophotography wherein the electrostatic image runs a continuous gradation of potential corresponding to a continuous gradation in the optical density of the subject to be reproduced.

In accordance with the present invention, there is provided an improved xerographic plate which is capable of reproducing an optical image faithfully and in pleasing tone rendition. In a process for which this improved member is particularly useful, a xerographic member comprising a photoconductive insulating layer on a conductive backing is provided with a uniform electrostatic charge on its photoconductive layer and is then exposed to a light image. On exposure to the light image, the photoconductive layer becomes conductive in selective areas corresponding to the bright area of the image, and the electrostatic charge is dissipated preferentially in these areas. To meet the high requirements of continuoustone electrophotography, it is essential that the electrophotographically-sensitive member be capable of accepting a uniform charge both in terms of uniform surface potential and in terms of total charge per unit area, and it is further essential that the photoconductivity of the sensitive member be uniform throughout the area and be generally proportional to the light intensity. Without these strictly exact characteristics, adequate continuous gradation of tone is unlikely, and the sensitive member would be suitable only for line copy reproduction or for relatively crude continuous-tone work.

It is, therefore, an object of this invention to provide an improved xerographically-sensitive member suitable for fine grade continuous-tone reproduction, and to provide a process for the preparation of said improved member.

It is a further object of this invention to provide an improved electrophotographic member comprising a photoconductive layer on a conductive backing, this member being of high uniformity suitable for fine continuoustone reproduction, and to provide a process for the preparation of such improved member.

In the production of a xerographic member suitable for electrophotography, any backing may be provided for this photoconductive insulating material which posall) ice

sesses greater conductivity than the dark conductivity of the photoconductive insulating layer, as stated in the aforesaid Carlson Patent 2,297,691, wherein reference is made to the use of metal plate backings such as zinc, aluminum, and brass. This patent also mentions the employment of other backings such as paper, glass, plastic or other sheets which may be impregnated or coated with a conductive material such as a conductive metal, metallicconductive compounds, carbon and the like, both with and without some binder material. In the xerographic member, the backing serves the primary purpose of providing a conductive material in continuous, electrically coupled relation with the layer of photoconductive insulating material whereby wherever the layer of photoconductive insulating material is rendered conductive responsive to exposure of the photoconductive insulating material to light, and to the extent of such exposure,

an electrostatic charge previously imposed on the layer of photoconductive insulating material may become dissipated with resultant production of an electrostatic image. Accordingly, many other backing plates may be employed in addition to those that have been exemplified. However, especially in the case of continuous tone reproduction, it is important that the electrical coupling between the backing and the layer of photoconductive insulating material be highly uniform throughout the area of the electrostatic latent image. Moreover, it is important that an original electrostatic charge be retained to the maximum extent possible and with maximum uniformity of retained charge during the interval between the charging of a xerographic member and the exposure of the member to light for the production of the desired photoelectric image.

The aforesaid Carlson Patent No. 2,297,691 gives a number of examples of photoconductive insulating materials which may be utilized in a xerographic member. Among them are sulfur, anthracene, anthraquinone, melted mixtures of sulfur and selenium with the sulfur predominating, and certain mixtures of sulfur with anthracene or linseed oil. More recently it has been proposed to employ as, and for, the layer of photoconductive insulating material, a layer of substantially pure selenium that has been produced under such conditions that the selenium is not in the semi-conductor metallic state mentioned in the said Carlson patent but is in an amorphous, vitreous, photoconductive insulator state, such as that which results from the procedure disclosed in application Serial No. 221,042, filed April 14, 1951, entitled Xerographic Member, and Method and Apparatus for the Production Thereof (now Patent No. 2,753,278, issued July 3, 1956). Such vitreous selenium photoconductive insulating material is considerably more electroconductively responsive than the materials mentioned in the Carlson patent, thus making it possible to utilize shorter periods of light exposure or less intense light exposure whereby greater all-around utility and suitability for continuous-tone are afforded. Accordingly, it is preferable to employ selenium for the photoconductive insulating material although in practicing this invention essentially the same utilization of photoconductive insulating properties is involved in the production of a photoelectric latent image, whether selenium is usedor some other photoconductive insulating material. The selenium that is used is substantially pure and does not contain the small amount of halide that is essentially present to impart conductive characteristics when selenium is used for other 3 purposes wherein substantial conductivity is a required characteristic.

Features of this invention involve the cleaning of a surface area of an electroconductive backing member which is free of substantially all foreign matter, thereby leaving the surface area substantially uniformly throughout so as to be that of the substance of which the backing member is composed, and while the surface is freed of all foreign matter, as aforesaid, causing a film of insulating material to be distributed over the surface area in directly adherent relation therewith with essential uniformity of, both composition and thickness. An essentially uniform layer of photoconductive insulating material thereafter is applied to the film of insulating material. The film of insulating material is so distributed in relation to the elements on the opposite sides thereof that the xerographic plate as a whole has a substantially decreased dark decay rate as compared with a corresponding plate wherein the surface of the backing member has been similarly cleaned without, however, applying the film of insulating material. At the same time, the plate permits effective discharge of an electrostatic charge upon increase in the conductivity of the layer of photoconductive insulating material responsive to exposure to light or other radiation that is effective to impart increased conductivity to the photoconductive insulating material that is used. In preferred embodiments of this invention, the intervening barrier film does not substantially decrease the rate of light decay of the plate or otherwise affect the responsiveness of the plate to light exposure and may actually increase the rate of light decay.

By utilization of this invention, very significant improvements in xerography have been obtained, especially in the attainment of improved continuous-tone reproductions. The sensitivity of a xerographic plate to light and to the development of electrostatic image contrasts is impaired by loss of an initially imparted electrostatic charge and, inasmuch as there is always a substantial interval of time between the imparting of the charge and the light exposure whereby the latent electrostatic image is produced and between the light exposure and development of the plate, the decrease in dark decay rate that is obtained according to this invention is of major practical significance as regards its effect on the quality of the reproductions produced. Moreover, both any dark decay as may occur and the light decay response to illumination are caused to be more uniform throughout the surface area that has been treated according to this invention with the result that the electrostatic image that is produced more nearly parallels the differences in illumination and avoids inaccuracies due to the effect onvthe electrostatic image of factors such as non-uniform charge retention and non-uniform response to illumination.

In the drawing the figure is a diagrammatic representation partially in section of an improved xerographic member according to this invention. In the figure there is shown a new xerographic member comprising a backing plate 11 having on its surface an extremely thin interfacial layer 12 with a top layer of photoconductive vitreous-appearing selenium 13.

The general nature and scope of the invention having been set forth, the following specific examples are present in illustration but not in limitation of the invention, and it is to be understood that the invention is limited only by the appended claims.

Example I Two brass test plates were thoroughly cleaned by treating each with successive applications of 1.2 N hydrochloric acid, isopropyl alcohol, and hot isopropyl alcohol vapor. To the surface of one of the cleaned plates a film was applied consisting of Lucite 46, which is a trade name of E. I. du Pont de Nemours & Co. for a polymeric butyl methacrylate, by dipping the plate in a toluene solution of the Lucite, the resulting film being about 3 microns in Initial Dark Light Interracial layer potent. decay, sensitivity Residual (positive), percent (400 m potent., v.

Lucite (3 555 0. 72 17. 5 26 None (control) 500 4 17. 5 25 In the foregoing table and elsewhere herein, dark decay expressed in percent is computed as follows:

where V is the initial potential in volts and V, is the potential after 30 minutes storage in the dark.

In the foregoing table and elsewhere herein, except as noted otherwise, the light sensitivity was measured at 400 millimircons using light intensity of 0.03 microwatt per square centimeter, the sensitivity value S being computed as follows:

Percent dark decay= V l/3 2 T los where T is the time in seconds required for the plate potential to decay under the light exposure to one-half of its initial value, V is the initial potential and I is the light intensity.

Example 11 Two brass plates were prepared as described in Example I and were placed in a common evaporator having separate shutters for screening each of the plates. While maintaining about 1 micron mercury pressure, a film of sulfur substantially 1.5 microns in thickness was applied to one of the plates while the other plate was screened by its shutter. Thereafter, without breaking the vacuum, a coating about 45 microns in thickness of amorphous selenium was applied at about 73 C. to both of the plates. The plate having the sulfur interfacial film exhibited much improved resistance to dark decay while possessing satisfactory light-sensitivity, as evidenced by the following data:

Example Ill Two brass plates were prepared as described in Example II except that the sulfur interfacial film was applied so as to be substantially 3 microns in thickness. As evidenced by the following data, the increased thickness of the sulfur interfacial film resulted in an increase in the resistance to dark decay without adversely affecting lightsensitivity:

Example IV Two brass plates were prepared as described in Example I, and each had successively applied thereto an interfacial film of gallium triselenide followed by the deposition of a coating of amorphous selenium following the manner described in Examples II and III. On one plate the interfacial film of gallium selenide was about 1 micron in thickness and on the other plate it was between 2 and 3 microns in thickness. Each of the plates exhibited high resistance to dark decay while possessing satisfactory light-sensitivity, as evidenced by the following data:

Two aluminum plates were thoroughly cleaned, first using isopropyl alcohol and then hot isopropyl alcohol vapor. One of the cleaned aluminum plates was coated with a Lucite film about 3 microns in thickness as described above in Example I. The interfacial film of Lucite resulted in a very good improvement in resistance to dark decay without impairing light-sensitivity, as evidenced by the following data:

Initial Dark Light Interracial layer potent. decay, sensitivity Residual (positive), percent (400 m potent., v.

Lucite 46 (3 459 3. 9 19 25 None (control) 430 18. 5 16 Example VI Two brass plates which had been cleaned as described above in Example I were placed in a vacuum evaporator and while maintaining a pressure of about 1 micron of mercury each of the plates had applied thereto by vacuum deposition a film of aluminum of about .3 micron in thickness. Without breaking the vacuum, one of the aluminum coated plates had deposited thereon a film of sulfur about 1 micron in thickness. Thereafter each of the plates had deposited thereon a coating about microns in thickness of amorphous selenium. In this instance also there was a pronounced improvement in resistance to dark decay while light-sensitivity remained high, as evidenced by the following data:

Initial Dark Light Interracial layer potent. decay, sensitivity Residual (positive), percent (400 m potent., v.

S (1 567 2.6 13. 2 14 None (control) 500 11 16 28 Example VII some increase in light-sensitivity. The data in connection with this example appears below:

Example VIII A film of gold about 1 micron in thickness was applied by vacuum evaporation to each of two clean brass plates. One of the plates then was provided with a Lucite film about 3 microns in thickness, as described above in Example I. Thereafter both plates were provided with a coating of amorphous selenium. The plates thus prepared when tested exhibited much improved resistance to dark decay and the light-sensitivity was virtually unaffected by the interfacial layer. In this instance the plates were also tested for fatigue and much improved resistance to fatigue was noted in the case of the plate having the interfacial Lucite film. The data in connection with this example is as follows:

Initial potent. Dark de- Fatigue, Light sen- Interfaeial layer (positive), eay, percent percent sitivity v. (450 m Lucite 46 (3 450 11 9.1 8.2 None (control) 100 32 32 8. 3

Fatigue as given in the above table and elsewhere herein is obtained by dividing the difference in voltage before and after exposures to radiation (in each case taken three minutes after charging) by the voltage on the plate before exposure (three minutes after charging), the value of the ratio being expressed as a percentage.

Example 1X The procedure of Example VII was repeated except for the vacuum deposition of a film of indium on the brass plates instead of gold, the results of this test being as follows:

Example V was repeated except that the plates were subjected to a negative potential rather than a positive potential and the following data was obtained:

Initial Light sensi- Interfacial layer potent. Dark decay, tivity (negative), v. percent (400 m Lucite 46 (3 500 45 11 None (control) 30 90 1 Example XI Example VI was repeated except that the plates were subjected to a negative potential rather than a positive potential and the following data was obtained:

Initial Light sensi- Interfacial layer potent. Dark decay, tivity (negative), v. percent (400 m Lucite 46 (3 500 54 6. 3 None (control) 70 0.3

7 Example XII The plate of Example I having the Lucite interfacial layer and the plate of Example III having the sulfur interfacial layer were each subjected to a uniform electrostatic charge comparable to that imposed in ordinary commercial xerography. Both plates were then stored in the dark for three days. At the end of this period each of the plates was exposed to a light image and was developed using normal xerographic procedures. The quality of the developed image was fully equivalent to that obtained using conventional plates when such plates were exposed and developed immediately after charging. The capacity thus exhibited for resisting dark decay for three days represents a very great improvement as compared with conventional plates previously employed in xerographv.

The function of the interfacial layer is not known with certainty but it is believed that in addition to protecting the surface from chemical action immediately prior to the coating operation it also acts as an insulator or at least as a barrier between the layer of photoconductive insulating material and the electroconductive substance of which the surface of the backing is composed. It is known, for example, that in vitreous selenium the mechanism of charged migration is attributable to the migration of positive holes, that is, such electrical conductivity as it possesses occurs in the form of the travel of apparent electron spaces or holes within the layer rather than by the travel of free electrons through the layer. It also is known that the preferred xerographic steps employed in connection with selenium as the photoconduetive insulating material include imparting a positive polarity surface charge to the photoconductive layer, which positive charge is dissipated upon exposure to light and is believed to be conductive through the layer to the conductive base plate or backing. Basing theory upon these facts, it is believed and understood that the interfacial film or layer of insulating material that, according to this invention, is provided between the electroconductive base plate and the layer of selenium or other photoconductive insulating material serves to prevent premature transmission of the electrons to the photoconductive layer prior to its exposure to light. Whether this theory is correct or not, it has been found that in the absence of the application of the interfacial film of insulating material the resulting xerographic plate is characterized by excessive loss of charge in the absence of illumination resulting in excessive loss of charge prior to and subsequent to exposure to the light image. The direct effect of such loss of charge is a weakened electrostatic irn-age and generally an image which is somewhat mottled and, therefore, unsuited for continuous-tone electrophotography.

The interfacial film which is employed according to this invention is composed of insulating material in contrast with the electroconductive substance presented at the surface of the base plate. The insulating material of which the interfacial film is composed also has less conductivity than the dark conductivity of the overlying layer of photoconductive insulating material in the sense that when the plate is charged and is not exposed to activating radiation an insulating barrier is provided which lessens the rate at which the potential imposed on the layer of photoconductive insulating material becomes dissipated into the base plate. The insulating material of which the interfacial film is composed ordinarily is not a photoconductive insulating material whose conductivity is increased upon exposure to radiation. However, such increase in conductivity upon exposure of the plate to activating radiation is not inconsistent with the practice of this invention inasmuch as it is desirable rather than otherwise that an imposed charge be dissipated rapidly in .those areas that are exposed to activating radiation. Thus, sulfur possesses photoconductive insulating characteristics to a certain extent but may very advantageously be used as the material of the interfacial layer in combination with an overlying layer of photoconductive insulating material such as vitreous selenium. However, the invention contemplates the employment of an interfacial film or barrier layer composed of some material which'is different from the coating of photoconductive insulating material that is primarily used to receive the electrostatic charge and on which the electrostatic image is produced.

As used in the xerographic process, an electrostatic field is placed across the photoconductive insulating layer. The interfacial film or barrier layer defined in the instant invention is an insulating film in the sense that it prevents the injection of carriers from the base plate into the photoconductive insulating layer under the influence of the applied field. Thus, such a layer may act to prevent the injection of both positive and negative charge carriers. However, as xerographic plates are normally used with only one polarity of charging, the barrier layer may prevent the injection of only one polarity of charge carrier. Thus, a vitreous selenium plate is norm-ally used with a positive field applied to the surface thereof. Accordingly, the barrier layer should act to prevent the injection of negative charges from the backing member into the selenium. If the selenium containsla quantity of arsenic tri-sulfide, the resulting xerographic plate is normally used with negative sensitization. For such a photoconductive insulating layer the barrier layer should prevent the injection of positive charges or holes from the backing into the photoconductive layer. The minimum thickness of the barrier layer is determined by two factors: First, the difficulty of obtaining a uniform interfacial film, and, secondly, the fact that the film must not be so thin that tunneling occurs to such an extent as to obviate the effectiveness of the barrier layer. For most materials this means that the interfacial film should have a thickness of at least about 0.1 micron. For certain materials, there is apparently an ability to strongly and uniformly wet .the surface of the brass backing, thereby creating a highly uniform layer even under conditions wherein the layer is extremely thin. The layer in those cases is no more than about or so angstroms thick. The limiting factor on the maximum thickness of the barrier layer is generally determined by the maximum residual voltage that can be tolerated under conditions of use of the plate. The formula for determining this follows from the fact that the capacity of the photoconductor is to the capacity of the barrier layer as the initial potential applied to the plate is to the residual potential. Assuming an initial potential of 500 volts applied to a selenium photoconductor, then 500 divided by the residual voltage equals the thickness in microns of the selenium times the dielectric constant of the interlayer divided by the thickness of the insulator times the dielectric constant of the selenium. For a 50- micron selenium plate, and an interlayer material with a dielectric constant of about 3, then the residual voltage is equal to 20 times the thickness in microns of the interlayer. It can be seen from this proportion that the larger the dielectric constant of the interlayer, the greater the permissible thickness of the interlayer. In general, it is desirable that the interlayer not be thicker than about 5 microns.

Apart from the presence of the thin interfacial layer of insulating material, it is essential that the surface area of the base plate to be used for electrophotography be scrupulously cleaned prior to the coating operation so that complete absence of any other interfacial material is assured. Dirt, moisture, spots, finger prints and other surface marks on the surface of the base plate, including even marks so slight as to be substantially invisible to the naked eye, have been found to be detectable through the coating in the form of substantial defects in xerographic prints produced from the plate. Accordingly, the surface presented by the electroconductive substance of the base plate should be cleaned so as to be essentially free of all foreign matter other than the electroconductive substance of which the surface area is composed.

In order to maintain the uniformity of results essential to fine quality continuous-tone xerography, it also is necessary that the selenium layer be of substantially uniform thickness. In the absence of such uniformity, the xerographic results are impaired in two ways. In the first place, the charge acceptance of the selenium layer is uneven so that a charge of non-uniform potential will be placed on the layer and this charge, furthermore, will be dissipated at a non-uniform rate. In addition, because of the varying distance between the surface of the base plate and the surface of the layer of photoconductive insulating material, a varying charge density, or coulorrrbs per unit area, will be present even if the potential is made uniform, with the result that varying numbers of charged electroscopic particles will be required to neutralize the charge by deposit on the surface. It is, therefore, apparent that uniformity of structure is critically essential to the fine quality demanded for continuous-tone reproduction.

In prior research on xerographic processes and materials it had been found that gross permanent defects frequently appeared in vitreous selenium xerographic layers, these defects being apparent after the layer had been charged to a potential approximating its maximum charge acceptance or insulation breakdown potential. These defects, appearing as dots about the size of the head of a pin, were characterized by inability to accept and retain an electrostatic charge, and were of such a nature as to render the member unsuitable even for line reproduction after a relatively large number of the spots had accumulated on the surface. Later, when these gross defects had been substantially completely eliminated by improvements and methods other than those of the present invention, it was found that other fine defects were detectable, these fine defects being substantially unobjectionable in line reproduction but making the member inferior for continuous-tone reproduction. In the presently preferred xerographic procedures, a xerographic member or plate is charged to a positive polarity potential of about 100 volts and exposed to a continuous-tone light image, and under these conditions it has been found that a typical prior xerographic plate such as, for example, an aluminum plate of clean, degreased mirror finish aluminum (which inherently has a thin aluminum oxide layer thereon) with a ZO-micron vitreous selenium layer does acquire such fine defects Whereas the plate according to this invention does not acquire such defects.

In preferred practice of this invention, the coating of photoconductive insulating material preferably is composed of vitreous selenium. The photoconductive insulating layer should be between about and about 200 microns thick, the thickness selected generally depending on the conditions of use of the plate. For line copy work the selenium usually is about 20 microns thick and for continuous-tone work preferably is about 50 microns thick, although the selenium layer may be somewhat thinner, e.g., about microns, or may be up to about 80 microns in thickness. The selenium layer for the drums of automatic machines generally is about 50 microns thick. For X-ray work the selenium layer usually is about 80 to 160 microns thick. While the nature of the selenium layer has been described as vitreous, the exact molecular structure is not known, the term being used as descriptive of its physical appearance and it is not desired to limit the use of selenium to any crystalline or allotropic form. It is believed that the selenium is present substantially in an amorphous form containing minor proportions of a crystalline form of selenium, although it is not desired to restrict this invention to the presence of such a mixture of forms. It is, therefore, to be understood that the various crystalline or amorphous structures included in the vitreous-appearing form of selenium are likewise to be included in the meaning of the term vitreous as used herein and in the claims. It likewise is to be understood that the term selenium includes not only pure selenium but also selenium that may be modified by a controlled amount of an additive that is consistent with retention of useful photo-insulating effectiveness. While the use of vitreous selenium is preferable, nevertheless xerographic plates using other types of photoconductive insulating material are improved by the employment of the interfacial film or barrier layer according to this invention. Moreover, while excellent results have been obtained wherein the base plate presents a surface composed of brass, nevertheless other electroconductive substances may be used so long as the surface thereof is free of visible imperfections and is essentially free of all foreign matter other than the electroconductive substance of which the surface area of the base plate is composed.

The presence of the barrier layer between the photoconductive insulating layer and the conductive backing not only reduces dark decay but is also believed to substantially reduce the buildup of trapped charges upon repeated use. Upon exposure to activating radiation, hole-electron pairs are created at the point of absorption of the photons. In the case of selenium this is on or close to the surface. The electrostatic field created by the sensitizing charges on the surface of the selenium cause one polarity of charge carriers to migrate through the selenium to the conductive backing. As they approach the backing, charges of opposite polarity are injected from the backing to migrate toward the surface. Most photoconductive insulators, such as selenium, cadmium sulfide, cadmium selenide, etc., have a long range for only one range of charge carrier. Thus, where both polarities of charge carriers are injected into the photoconductor, the short range carriers are trapped building up a strong residual potential as the plate is reused. The presence of a barrier interlayer according to the instant invention, by preventing injection of carriers from the backing, assures that only those carriers generated by the incident radiation will be present thus reducing the possibilities for trapping.

What is claimed is:

1. A xerographic plate comprising:

(a) a metallic backing member having a substantially smooth surface;

(b) a barrier film on said surface having a thickness of up to about 3 microns, said barrier film comprismg a material selected from the group consisting of sulfur and gallium t-riselenide; and,

(c) a layer of substantially uniform thickness of photoconductive selenium on said film, said layer having a thickness of at least 20 microns.

2. The xerographic plate of claim 1 wherein said barrier film comprises sulfur having a thickness in the range of 1.5 to about 3 microns.

3. The xerographic plate of claim 1 wherein said barr1er layer comprises gallium triselenide and is about 1 micron in thickness.

4. The xerographic plate of claim 1 wherein said metallic backing member comprises brass.

5. The xerographic plate of claim 1 wherein said metallic backing member comprises aluminum.

References Cited by the Examiner UNITED STATES PATENTS 2,739,243 3/1956 Sheldon 96-1 2,787,745 4/1957 Strosche et al. 317-241 2,901,348 8/1959 Dessauer et al. 96-1" 2,937,944 5/1960 Van Dorn et a1 96-l 2,956,218 10/1960 Kleinle et al. 317-241 FOREIGN PATENTS 755,683 8/1956 Great Britain.

NORMAN G. TORCHIN, Primary Examiner. C. VAN HORN, Assistant Examiner. 

1. A XEROGRAPHIC PLATE COMPRISING: (A) A METALLIC BACKING MEMBER HAVING A SUBSTANTIALLY SMOOTH SURFACE; (B) A BARRIER FILM ON SAID SURFACE HAVING A THICKNESS OF UP TO ABOUT 3 MICRONS, SAID BARRIER FILM COMPRISING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF SULFUR AND GALLIUM TRISELENIDE; AND, (C) A LAYER OF SUBSTANTIALLY UNIFORM THICKNESS OF PHOTOCONDUCTIVE SELENIUM ON SAID FILM, SAID LAYER HAVING A THICKNESS OF AT LEAST 20 MICRONS. 